The invention and its various independent aspects, uses and objects are particularly useful and beneficial in the field of wireless communications, including in Wireless Local Area Networks (WLANs). Wireless devices often are deployed in environments that are electrically noisy and not optimal for wireless communications. For example, most homes and work places include many electronic devices resulting in an electronically noisy environment that may interfere with communications, such as microwave ovens, garage door openers, radios, television sets, computer systems, etc. The communication medium between wireless devices may change constantly. Most environments include multiple reflective surfaces and corners, creating multi-path noise. Also, movement of items or devices or the like, such as hands, bodies, jewelry, mouse pointers, etc. or activation of electronic devices, such as cooling fans or the like, affects the overall wireless communication path and potentially degrades wireless communication performance. Organizations, such as IEEE, have drafted and implemented standards to facilitate deployment of and enhance the efficacy of wireless communications systems and components.
The Institute of Electrical and Electronics Engineers, Inc. (IEEE) 802.11 specification falls under the IEEE 802 family of specifications for Local Area Network (LAN) technologies and is in essence a link layer that uses IEEE 802.2 Logical Link Control (LLC) encapsulation. The IEEE 802.11 specification includes a Media Access Control (MAC) sublayer on top of 802.11a/b/g physical layers (PHY) and represents a family of standards for wireless local area networks (WLAN) in the unlicensed ISM bands of 2.4 and 5 Gigahertz (GHz) bands. The IEEE 802.11b standard specifies a high rate direct sequence spread spectrum (HR/DSSS) with a chip rate of 11 Megahertz (MHz) and defines various data rates in the 2.4 GHz band, including data rates of 1, 2, 5.5 and 11 Megabits per second (Mbps). The IEEE 802.11a standard defines different and higher data rates of 6, 12, 18, 24, 36 and 54 Mbps in the 5 GHz band. It is noted that systems implemented according to the 802.11a and 802.11b standards are incompatible and will not work together. The IEEE 802.11g standard is, in essence, an extension to 802.11b that broadens data rates to 54 Mbps within the 2.4 GHz band using OFDM. Because of backward compatibility, an 802.11b radio card interfaces directly with an 802.11g access point (AP) (and vice versa) at 11 Mbps or lower depending on range.
A radio configured in accordance with IEEE 802.11a or 802.11g standards employs Orthogonal Frequency Division Multiplexing (OFDM) modulation in which a stream of data is transmitted over multiple small frequency sub-channels. In the OFDM configuration, multiple sub-carrier signals are incorporated within each OFDM symbol. Data is incorporated on each data tone using a selected modulation scheme, such as Binary Phase Shift Keying (BPSK), Quadrature PSK (QPSK), 16 Quadrature Amplitude Modulation (QAM), and/or 64 QAM. Each of the modulation schemes employs a corresponding constellation map with variable constellation points corresponding to a corresponding variable number of bits for achieving the various data rates. For example, BPSK is used for 6 or 9 Mbps, QPSK is used for 12 or 18 Mbps, 16 QAM is used for 24 or 36 Mbps, and 64 QAM is used for 48 or 54 Mbps. The encoding process employs a quadrature generation technique and provides in-phase (I) and quadrature (Q) signals on respective I and Q channels.
The IEEE 802.11a standard employs OFDM using 20 megahertz (MHz) wide channels in the 5 gigahertz (GHz) radio frequency (RF) band. Also, by way of example, wireless standards based on using OFDM with 10 MHz channels include the IEEE 802.11j standard for use in Japan and the DSRC standardization (Dedicated Short Range Communications). DSRC is a communications approach to allowing short range communications between vehicles and the roadside for a variety of purposes, such as electronic toll collection, intersection collision avoidance, transit or emergency vehicle signal priority, electronic parking payments, and commercial vehicle clearance and safety inspections. IEEE 802.11j (or, “11j”) and DSRC have both described achieving OFDM with 10 MHz channels by using a clock at one-half the rate the 802.11a OFDM clock, or 10 MHz kernel sampling.
IEEE 802.11n standard has been proposed to provide higher throughput and calls for rates of at least 100 Mbits/second. This performance would be measured at the interface between the 802.11 MAC and higher layers, rather than at the PHY layer, to evaluate the net data rate experienced by the user. The net data rate in WLANs is significantly affected by sources of overhead within the 802.11 protocol, e.g., packet preambles, acknowledgements, contention windows and interframe-spacing. As a result, for example, although the 802.11b standard specifies a peak physical-layer rate of 11 Mbits/s, the typical net peak delivered is 5 to 6 Mbits/s. Also, although the 802.11a and 802.11g standards specify a peak PHY data rate of 54 Mbits/s, the typical net peak delivered is 20 to 24 Mbits/s. Accordingly, the 802.11n high throughput standard represents a four- to five-fold increase in actual throughput over that achievable with 802.11a/g. 802.11n specifies backward compatibility with legacy 802.11a/g deployments.
IEEE 802.11 networks typically consist of four physical components, including a distribution system, access points (APs), wireless media, and mobile stations comprising a basic service set (BSS). The mobile stations of a BSS are computing stations, such as notebook computers, PDAs, mobile telephones and other network devices, e.g., printers, facsimile machines, scanners, copiers, hubs, routers, switches, etc, that communicate with each other across one or more APs, which in turn communicate with each other over a distribution system. The communication between the BSS stations and the APs may form a basic service area (BSA) and occurs over a wireless medium. The communication between the APs and each other and the distribution system may be over a plurality of communications media, including wireless and wired media. The APs essentially perform a bridging function. Further, the distribution system may comprise or be in communication with a plurality of communications systems over a plurality of media. Also, within the realm of 802.11 is an independent BSS wherein mobile stations communicate directly with one another. Accordingly, the smallest 802.11 network may consist of two mobile stations communicating with one another. Also, multiple BSSs may be linked together to form extended service sets (ESSs).
In operation, a short training period is typically included at the start of each transmission, including Short Syncs and two Long Syncs (LS) appended at the front end of each transmitted frame. The Long Syncs provide a reference amplitude and phase for each of the active subchannels. The Long Syncs may be averaged together to reduce the noise in the received reference values. After the Long Syncs have been received, each sub-channel received symbol is multiplied by the inverse of the reference amplitude and the conjugate of the reference phase (when expressed as a complex unit vector) for that sub-channel. This removes most of the amplitude and phase distortion that has occurred between the transmitter and the receiver.
Optimum soft-decisions should be Signal-to-Noise Ratio (SNR) weighted. The Long Syncs have been used to generate LLR (log likelihood ratio) weights to correctly weight soft-decisions going into an error-correcting decoder, such as a Viterbi decoder or the like. Given a flat noise floor, the use of LLR weights translates into a signal-power weighting. These LLR weights have been the signal power determined in each sub-channel of the Long Syncs. Using the LLR weights improves soft-decisions and reduces transmission errors.
A first problem is that the received reference values are usually degraded by noise, which is an unavoidable consequence of radio transmission. The reference information provided during the training phase (e.g., in the Long Syncs) is known by the receiver, so that a significant amount of this noise can be determined. A second problem, however, is that the signal amplitude and phase distortion may change over time, from the start of transmission of each frame to the end of the frame, making the initial channel estimate information obsolete and inaccurate towards the end of each frame. Both of these problems increase the probability of error when receiving a frame, due to both signal equalization errors and soft-decision weighting errors.
Legacy radios were designed with several assumptions. The channel was assumed to be relatively stable. The frames were bursty in nature and relatively short, so that it was assumed that the wireless channel did not significantly change over the duration of each frame. The initial channel estimate information determined at the start of each frame was assumed to be sufficiently accurate for that frame. Presently, however, there is a greater emphasis on mobility and/or accuracy. Mobility results in a changing environment that could result in significant changes in the channel during each frame. Even in a stable environment, improved accuracy can improve transmission speed and enable a higher transmission rate with a lower packet error rate (PER). It is desired to improve channel estimation to enable mobile application and/or higher transmission rates.